Investigation of the mechanism of dissociation of glycolaldehyde dimer (2,5-dihydroxy-1,4-dioxane) by FTIR spectroscopy

Article abstract of DOI:10.1016/S0008-6215(98)00129-3

Glycolaldehyde represents the simplest α-hydroxycarbonyl moiety - a common structural feature of reducing sugars. It exists in solid state, only in crystalline dimeric form as 2,5-dihydroxy-1,4-dioxane. However, in solution phase or during heating, it dissociates into different dimeric and monomeric forms. FTIR spectroscopy was used to study the effect of temperature, pH and solvent on the dissociation and chemical transformations of glycolaldehyde. The infrared spectra were recorded in different solvents as a function of time and temperature (both during heating and cooling cycles) between 30 and 85°C. During heating, glycolaldehyde cyclic dimer generated two bands in the carbonyl region, one at 1744 cm^-1 and the other at 1728 cm^-1. These bands increased during the heating cycle and decreased during the cooling cycle. The data indicated that the glycolaldehyde cyclic dimer (2,5-dihydroxy-1,4-dioxane) undergoes a ring opening to form the acyclic dimer (1728 cm^-1) that can recyclize into the 2-hydroxymethyl-4-hydroxy-1,3-dioxolane structure. The acyclic dimer can also dissociate into monomeric glycoladehyde (1744 cm^-1) in equilibrium with the enediol form (1703 cm^-1). There is evidence to indicate oxidation of glycolaldehyde into glycolic acid during heating, in either neutral or basic aqueous solutions.

Full text of DOI:10.1016/S0008-6215(98)00129-3

Carbohydrate Research 309 (1998) 31±38
Investigation of the mechanism of dissociation
of glycolaldehyde dimer (2,5-dihydroxy-1,4-
dioxane) by FTIR spectroscopy
Varoujan A. Yaylayan*, Susan Harty-Majors, Ashraf A. Ismail
McGill University, Department of Food Science and Agricultural Chemistry, 21,111 Lakeshore, Ste.
Anne de Bellevue, Quebec, H9X 3V9, Canada
Received 25 March 1998; accepted 30 April 1998
Abstract
Glycolaldehyde represents the simplest ꢀ-hydroxycarbonyl moiety±a common structural feature
of reducing sugars. It exists in solid state, only in crystalline dimeric form as 2,5-dihydroxy-1,4-
dioxane. However, in solution phase or during heating, it dissociates into di€erent dimeric and
monomeric forms. FTIR spectroscopy was used to study the e€ect of temperature, pH and solvent
on the dissociation and chemical transformations of glycolaldehyde. The infrared spectra were
recorded in di€erent solvents as a function of time and temperature (both during heating and
ꢀ
cooling cycles) between 30 and 85 C. During heating, glycolaldehyde cyclic dimer generated two
bands in the carbonyl region, one at 1744 cm 1 and the other at 1728 cm 1. These bands increased
during the heating cycle and decreased during the cooling cycle. The data indicated that the gly-
colaldehyde cyclic dimer (2,5-dihydroxy-1,4-dioxane) undergoes a ring opening to form the acyclic
dimer (1728 cm 1) that can recyclize into the 2-hydroxymethyl-4-hydroxy-1,3-dioxolane structure.
The acyclic dimer can also dissociate into monomeric glycoladehyde (1744 cm 1) in equilibrium
with the enediol form (1703 cm 1). There is evidence to indicate oxidation of glycolaldehyde into
glycolic acid during heating, in either neutral or basic aqueous solutions. # 1998 Elsevier Science
Ltd. All rights reserved
Keywords: FTIR; Glycolaldehyde; Glyoxal; ꢀ-Hydroxycarbonyl moiety; Dissociation mechanism; Autoxidation
1. Introduction
undergo Amadori rearrangement in the presence of
amines, that eventually transforms aldehydo sugars
into their ꢀ-amino keto derivatives (1e). The phy-
sical and chemical properties of reducing sugars in
solution depend on the relative concentrations of
di€erent monomeric and dimeric forms originating
from the ꢀ-hydroxy carbonyl-moiety. Their biolo-
gical properties can also have similar dependence
[1]. It appears that the enediol forms of certain
sugar derivatives in biological systems are the
The ꢀ-hydroxycarbonyl moiety (1a in Scheme 1)
imparts to reducing sugars exceptional molecular
¯exibility, manifested in such processes as enoliza-
tion (1b and 1c), isomerization (1d) and dimeriza-
tion (1f±1h). In addition, it enables these sugars to
* Corresponding author. Tel.: (514)-398-7918; fax: (514)-
398-7977; e-mail: czd7@musica.mcgill.ca
0008-6215/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved
PII S0008-6215(98)00129-3

32
V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
Scheme 1. Chemical transformations of the ꢀ-hydroxycarbonyl moiety of reducing sugars.
active forms with which enzymes react [2,3]. Ene-
diol or enol forms are also known to play an
important role in metal-catalyzed oxidative degra-
dation of reducing sugars [4]. Such complexity in
the population of reducing sugars in solution,
makes their study a dicult proposition, especially
for hexoses and pentoses, where the presence of
more stable furanose and pyranose forms renders
the ꢀ-hydroxycarbonyl moiety dicult to detect at
room temperature due to their low concentrations
(<1%). Solvent interference, especially from
water, makes the detection of aldehydo forms even
more dicult due to hydration (1i). FTIR spectro-
scopy has been used to study the e€ect of tem-
perature [5] on acyclic forms of d-fructose,
mutarotation of d-glucose and d-fructose [6], and
enolization and carbonyl group migration in selec-
ted reducing sugars [7]. Simple ꢀ-hydroxyl carbo-
nyl compounds such as glycolaldehyde (1a, R=H),
glyceraldehyde (1d, R=CH2OH), and dihydroxy-
acetone (1a, R=CH2OH) have the tendency to
from stable cyclic (1g and 1h) and acyclic (1f)
dimers that dissociate into monomeric forms on
standing in dilute aqueous solutions [8,9] or by
increasing the temperature. Such dimer to mono-
mer conversions can be followed by observing an
increase in the intensity of the carbonyl band using
FTIR spectroscopy. FTIR spectroscopy is ideally
suited to study the di€erent monomeric and
dimeric forms of compounds possessing the ꢀ-
hydroxycarbonyl moiety 1a. The forms derived
from 1a absorb at di€erent frequencies such as
1
carbonyls, between 1700 and 1750 cm , enediols
1
between 1630 and 1700 cm and C±O±C between
1
950 and 1300 cm . Simple ꢀ-hydroxycarbonyl
compounds such as glycoladehyde (1a, R=H), 1-
hydroxyacetone (1a, R=CH3), 1-hydroxy-2-buta-
none (1a, R=CH2CH3) and glyceraldehyde (1d,
R=CH2OH), could be used to investigate the
ability of FTIR spectroscopy to monitor the

V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
33
molecular transformations in these compounds
under di€erent environmental conditions. In this
study, the e€ect of solvent, pH, and temperature
on the dissociation of dimeric glycolaldehyde (1h,
R=H) was investigated using FTIR spectroscopy.
form, but has not been detected by NMR studies.
To investigate the ability of FTIR to detect the
di€erent monomeric and dimeric forms, glyco-
laldehyde was analyzed in di€erent solvents and at
di€erent pH and temperatures. The data obtained
from these studies revealed the following observa-
tions: (a) when glycolaldehyde dimer (2a) was stu-
died at room temperature, either in solution phase
(in D2O or dioxane) or as a melt (Fig. 1), two car-
2. Results and discussion
1
General observations.ÐIn the solid state, glyco-
laldehyde is known [8] to exist in dimeric form as
2,5-dihydroxy-1,4-dioxane (2a) as shown in
Scheme 2. In the molten state, it exists as a mixture
of di€erent monomeric and dimeric forms [10]. In
the vapor phase, only the monomers exist in cis
orientation exhibiting a single carbonyl absorption
bonyl absorption bands, one at 1744 cm and the
1
1
other at 1732 cm (shifted to 1728 cm in D2O
1
and 1736 cm in dioxane), were detected, along
1
with a weaker band at 1703 cm . These bands
increased during the heating cycle and decreased
during the cooling cycle, indicating their reversi-
1
bility (Fig. 2). The intensity of the 1744 cm band
1
1
band at 1753 cm [11]. In the solution phase, the
was higher than that of the 1728 cm band. (b) In
D2O, two additional bands emerged at 1673 and
dimer undergoes dissociation into monomeric
forms; however, the dissociation rate and the rela-
tive concentration of di€erent species depend on
the temperature and the type of solvent [11,12].
Studies performed by NMR spectroscopy [13] at
room temperature in D2O indicated the presence of
at least four molecular species (see Scheme 2): 2,5-
dihydroxy-1,4-dioxane (2a, 9%), 2-hydroxymethyl-
4-hydroxy-1,3-dioxolane (2c, 17%), monomeric
glycolaldehyde (2d, 4%), and hydrated monomer
(2f, 70%). In Me2SO, the equilibrium concentra-
tion of 2d remained the same, and the concentra-
tions of 2a and 2c increased to 36 and 60%,
respectively. However, the transformation of diox-
ane 2a to dioxolane 2c requires the intermediacy of
the acyclic dimer 2b that has been proposed [14] to
1
1593 cm
irreversibly with temperature. (c) In slightly basic
(Fig. 3). These new bands increased
1
D2O (0.1% NaOD), the band at 1673 cm dis-
1
appeared (Fig. 3), and the band at 1593 cm
increased irreversibly with temperature. In addi-
tion, the intensities of the bands at 1744 and
1703 cm 1 decreased continuously, both during the
heating and cooling cycles, whereas the intensity of
1
1728 cm band increased during the heating cycle
and decreased during the cooling cycle (Fig. 4). (d)
Under strongly basic conditions (20% NaOD or
100% triethylamine), all the bands mentioned
1
above disappeared except the band at 1593 cm ,
which became the predominant band. (e) Under
acidic conditions (1% DCl) and in the melt, the
Scheme 2. Mechanism of dissociation of glycoladehyde dimer. Percent composition in D2O at room temperature [13].

34
V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
1
bands at 1673 and 1593 cm were not detected.
1
However, the bands at 1744, 1728, and 1703 cm
increased during the heating cycle and decreased
during the cooling cycle.
Two important conclusions can be drawn from
these observations: one regarding the mechanism
1
Fig. 1. Absorption of the carꢀbonyl region (1700±1800 cm
)
of glycolaldehyde (melt at 90 C). Fourier self-deconvolution
(Á Á ÁÁ Á Á) (Bandwidth 10.5, Enhancement factor 1.6).
Fig. 3. Comparison of the absorption region between 1700
1
and 1600 cm of 20% glycolaldehyde in D2O (Ðб) and in
0.1% NaOD (Á Á Á).
ꢀ
ꢀ
Fig. 2. E€ect of heating (solid lines, 35±85 C at 10 C incre-
ments) and cooling (dotted lines, 85±35 ꢀC at 10 ꢀC incre-
ments) on the intensity of the carbonyl absorption bands
(1700±1800 cm 1) of glycolaldehyde melt.
1
Fig. 4. Absorption of the carbonyl region (1700±1760 cm
of a 20% glycolaldehyde solution (0.1% NaOD in D2O) at
)
ꢀ
ꢀ
ꢀ
. . .. ..
30 C (Ðб), 80 C (- - -) and at 30 C cooling (- - -).

V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
35
of dissociation of the glycoladehyde dimer, and the
second regarding its chemical stability in basic or
neutral aqueous solutions. The detection of two
carbonyl bands indicates the presence of another
carbonyl species in addition to the glycolaldehyde
initial imine adduct through similar rearrangement
due to lack of an ꢀ-hydroxyl group and is thus not
expected to generate a stable end product. A 20%
solution of the dimer 2a in D2O was heated in a
temperature-controlled cell in the absence anꢀd pre-
sence of glycine. The initial temperature (30 C) of
the cell was raised by 1 ꢀC per min, and every 5 min
the temperature was kept constant for 15 min to
record the spectra. During the heating cycle, both
carbonyl peaks increased in intensity in the absence
of glycine. However, in the presence of glycine, the
monomer, and the detection of the band at
1
1593 cm
indicates oxidation of glycoladehyde
into a carboxylate moiety such as glycolic acid or
glyoxylic acid.
Mechanism of the dissociation of glycolaldehyde
dimer.ÐThe presence of two reversible and tem-
perature sensitive carbonyl bands, in the FTIR
spectrum of glycolaldehyde dimer 2a, provides the
®rst evidence for the stepwise ring opening (see
Scheme 2) of the dioxane structure 2a to form the
acyclic dimer 2b, which dissociates into monomeric
form 2d or recyclizes into the dioxolane ring struc-
ture (2e). Apparently, this process is too fast for
the time scale of NMR to be detected, while the
time scale of FTIR allows the rapidly inter-
converting species to be detected. The process of
interconversion of the four species (2a±2d) in solu-
tion is similar to the mutarotation of reducing
sugars in which the acyclic form is in equilibrium
with cyclic furanose and pyranose forms. Tenta-
1
band at 1728 cm increased with the temperature,
1
unlike the band at 1744 cm which decreased in
intensity with the concomitant appearance and
1
increase of a new band at 1765 cm , consistent
with the formation of an Amadori adduct. The
sensitivities of the above-mentioned two bands to
changes in pH were also studied. Both bands were
stable under acidic pH as they increased during the
heating cycle and decreased during the cooling
cycle, similar to their behavior in neutral D2O.
However, at mildly basic pH, the band at
1
1744 cm
continuously decreased during both
1
cycles, whereas the band at 1728 cm increased
during the heating cycle and decreased during
cooling (Fig. 4). These observations are consistent
with the band assignments. Furthermore, a 20%
solution of trimeric glyoxal dihydrate (3, Scheme 3)
in D2O was also studied by FTIR and found to
exhibit a very weak carbonyl absorption band at
1
tively, the band at 1744 cm was assigned to the
1
monomer 2d, the band at 1728 cm to the acyclic
1
dimer (2b), and the band at 1703 cm
enediol.
to the
Con®rmation of band assignments.ÐThe initial
assignments of the two carbonyl bands were based
on the fact that in the presence of ꢀ-hydroxyl
groups, the stretching frequencies of carbonyl
bands are shifted to higher values relative to simple
alkyl substituted carbonyl compounds, provided
that the hydroxyl group can rotate to eclipse the
carbonyl group. The magnitude of this shift
depends on the torsional angle. This e€ect was
demonstrated in monosaccharides [7] by observing
a shift to a higher frequency in the carbonyl
absorption bands of di€erent sugars relative to
their ꢀ-deoxy derivatives. Accordingly, the peak at
1
1744 cm . Investigation by NMR spectroscopy
[15] of the principal species present in an aqueous
solution of glyoxal has indicated the presence of
hydrated monomer 3e, dioxane dimer 3b and two
additional dimers whose structures contained a
®ve-membered dioxolane rings (3h and 3f) as
shown in Scheme 3. However, no aldehydic pro-
tons were detected, although the formation and
further transformation of 3f into 3h requires the
presence of aldehydes 3d and 3g. All the carbonyl
species (3a, 3d, and 3g) originating from glyoxal
trimer in water, have an ꢀ-hydroxyl group and thus
are expected to show similar absorption fre-
quencies to that of monomeric glycoladehyde.
Additional supporting evidence for the band
assignments was provided from the solvent e€ect.
According to NMR studies [13], aprotic solvents
can increase the relative concentrations of the
dimeric versus monomeric forms, for example, in
Me2SO the percent composition of 2a and 2c
increased to 36% (from 9%) and 60% (from 17%),
respectively, relative to D2O. In dioxane, the order
1
1744 cm was assigned to the monomer that pos-
sesses a ꢀ-hydroxyl group. In gas phase, where
only monomers exist, the single carbonyl band
1
appeared at 1753 cm
[11]. To provide further
evidence for this assignment, the reactivities of the
two carbonyl bands were compared in the presence
of glycine. The reaction of the monomer with gly-
cine should produce a stable Amadori product
similar to 1e (R=H) with the appearance of a new
carbonyl peak, whereas 2b is unable to stabilize the

36
V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
Scheme 3. Proposed mechanism of dissociation of glyoxal trimer.
of the intensity of the two carbonyl bands at 1728
1
into a carboxylate moiety. To demonstrate the
irreversibility of the new bands, a 20% solution of
glycolaldehyde dimer in D2O was also subjected to
and 1744 cm was interchanged, where the band
1
at 1728 cm was more intense than the band at
1
ꢀ
1744 cm , which is consistent with the NMR
observations.
heatingꢀ(30±80 C) and subsequent cooling cycles
ꢀ
(80±30 C). The initial temperature (30 C) of the
Monitoring oxidation of glycoladehyde by FTIR
spectroscopy.ÐSimple aldehydes are known to
undergo autoxidation into their corresponding
carboxylic acids in the presence of water [16]. The
accepted mechanism involves the reaction of mole-
cular oxygen with the enediol moiety of the alde-
hyde to form ꢀ-hydroperoxides [17] and
subsequent loss of hydrogen peroxide [18]. A
freshly prepared solution (20%) of the crystalline
cell was raised by 1 ꢀC per min, and every 5 min the
temperature was kept constant for 15 min to record
ꢀ
the spectra. At 30 C the dimer solution showed
1
weak absorption bands at 1744 and 1728 cm .
During the heating cycle the existing bands and the
1
new emerging bands at 1703, 1673 and 1593 cm
increased with increasing temperature. However,
during cooling cycle the bands at 1744, 1728 and
1
1703 cm decreased and the bands at 1673 and
1
ꢀ
dimer 2a in D2O at 25 C, showed only weak car-
1
1593 cm continued to increase, demonstrating
the irreversibility of the process giving rise to these
bonyl absorption bands at 1744 and 1728 cm .
When the solution was monitored by FTIR over a
1
bands. In addition, during cooling, the 1728 cm
1
3 h period at 30 ꢀC, a new band at 1593 cm
emerged and increased slowly over time. Similarly,
band shifted slowly to 1725 cm
(see Fig. 6).
1
Under acidic pH or in dioxane the bands at 1673
and 1593 cm 1 did not appear.
a solution (20%) of the crystalline dimer 2a in D2O
ꢀ
was monitored at 70 C for 4 h by scanning the
FTIR spectra every 15 min. During the 4 h of
incubation, the intensities of the bands at 1744,
To con®rm the identity of the new product
formed, the FTIR spectra of the free and sodium
salts of the two possible carboxylic acids, glyoxylic
and glycolic acids, were acquired in D2O. The gly-
1728 and 1703 cm 1 decreased with the appearance
1
1
and increase of a new band at 1593 cm (see
Fig. 5), indicating the conversion of glycoladehyde
colic acid absorbed at 1725 cm and glycolate ion
1
at 1593 cm . One the other hand, the glyoxylic

V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
37
ꢀ
Fig. 6. E€ect of heating (dotted lines, 30±80 C) and cooling
(solid lines, 80±30 ꢀC) on the intensity of the absorption bands
Fig. 5. E€ect of time (0±4 h) on the intensity of the carbox-
1
ylate band at 1593 cm of a 20% solution of glycolaldehyde
ꢀ
1
(1750±1650 cm region) of a 20% solution of glycolaldehyde
dimer in D2O.
in D2O monitored at 70 C.
1
acid absorbed at 1735 cm and glyoxylate ion at
1
1577 cm , indicating autoxidation of glycolade-
3. Experimental
hyde into glycolic acid in aqueous solution. When
the pH of a freshly prepared solution of glycola-
dehyde dimer in distilled water was monitored at
Glycoladehyde dimer and glyoxal trimeric dihy-
drate were obtained from Aldrich Chemical Co.
Dioxane was purchased from Fisher Scienti®c.
D2O was obtained from MSD Isotopes (Montreal,
Canada). Solutions (20%) of the samples were
prepared in D2O or dioxane. The spectra were
acquired on an CaF2 IR cell with a 25 ꢁm Te¯on
ꢀ
45 C, the pH quickly dropped to pH 4 in 2 min
and continued to decrease to pH 3.6 during 1 h of
monitoring. Scheme 4 shows a simpli®ed proposal
for the autoxidation mechanism consistent with the
above observations.
Scheme 4. Proposed mechanism of autoxidation of glycolaldehyde into glycolic acid.

38
V.A. Yaylayan et al./Carbohydrate Research 309 (1998) 31±38
spacer. Glycolaldehyde melt was obtained by
microwave irradiation for 120 s in a domestic
microwave oven (700 W). The spectra of the melt
were acquired without spacer. Processing of the
FTIR data was performed using GRAMS/386
version 3.01 (Galactic Industries Corporation, 1994).
Temperature studies.ÐSample solutions in D2O
were placed in a CaF2 IR cell with a 25 ꢁm Te¯on
spacer. The temperature of the sample was regu-
lated by placing the IR cell in a temperature-con-
trolled cell holder. Infrared spectra were recorded
on a Nicolet 8210 Fourier-transform spectrometer,
purged with dry air, and equipped with a deuter-
ated triglycine sulfate (DTGS) detector. The initial
temperature of the cell was raised by 1 ꢀC/min, and
every 5 min the temperature was kept constant for
15 min to record the spectra. A total of 128 scans at
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